The Getty recently announced a new program in support of restriction-free sharing of public domain images with the launch of their "Open Content Program." As a first step toward making "all images of public domain artworks in the Getty’s collections" available, 4600 high-resolution images have been uploaded and are available free of charge. The images are embedded with metadata for identification purposes and Getty does request information from those who download the files, just so they know what the images are being used for.
The Open Content Program will see various updates in the future, with thousands more images being added from the museum's collection and from the Getty Research Institute's special collection once copyright policies have been reviewed. The initiative is described as an "educational imperative," one that will provide teachers, students, and others the ability to share and be creative with images without fear of copyright infringement.
To view and use the images, check out the Open Content page of their online database.
UPDATE October 22, 2013 8:27 p.m.: Getty has added another 5400 artwork images to their Open Content Program, putting the current total over the 10k mark.